Tools (moulds) made from tool steel are used for the forming of plastic articles, e.g. for injection moulding and compression moulding. These tools often are very large and, at the same time, they may have a very complicated design.
During the plastic forming operation, the tools are subjected to high stress: in the first place mechanical stress but also in the form of chemical attacks. This can cause different types of damages of the tools, above all of the following nature:
abrasion, PA1 plastic deformation (impressions), PA1 rupture (fatigue), and PA1 corrosion.
The features of the tool steel have significant importance for the resistance of the tools against these types of damages. In principle a perfect tool steel shall be hard, tough and corrosion resistant in order to produce plastic forming tools which have a high capacity and at the same time a good reliability.
Another important thing is that complicated tools shall be able to be manufactured in a reasonably simple manner, e.g. through cutting operations. This implies that the tool steel if possible should satisfy the following conditions:
It shall be soft (&lt;40 HRC) when the tool is being manufactured, i.e. in the starting condition.
It shall be possible to make the steel hard (&gt;45 HRC) by means of a simple heat treatment of the finished tool without any changes of the shape or of the dimensions of the tool which would require complicated adjustments.
If all these aspects are considered, the following combination of the desired features may be listed for the perfect tool steel for plastic forming:
1--Hardness&lt;40 HRC in the starting condition. PA0 2--Hardness&gt;45 HRC, preferably about 50 HRC, shall be achieved through a simple heat treatment. PA0 3--It shall be possible to provide an even hardness also in the case of very large dimensions (large size tools). PA0 4--The increase of the hardness shall be achieved without any complicating changes of shape or volume. PA0 5--The steel shall have a high corrosion resistance, i.e. be of the stainless type. PA0 6--The steel shall have a sufficient toughness. PA0 7--The steel shall be able to be afforded an extra good wear resistance through e.g. any simple surface treatment.
Since a good corrosion resistance is a primary requirement, a steel of this type has to be found within the category of steels which includes stainless steels, i.e. steels having a chromium content&gt;10%. There exist today a large number of more or less commercially established stainless steels. A thorough technical evaluation of the steel types which already exist can be summed up in the following way as far as the desired features are concerned (1-7 above):
Austenitic, ferritic, and ferritic-austenitic stainless steel grades do not have qualifications to fulfill the requirement as far as hardness is concerned (2), not even precipitation-hardenable variants.
Martensitic stainless steels based on carbon martensite, so called 13% chromium steels etc., have better conditions to provide the desired combination of features. Due to the fact that they have to be hardened and tempered in order to fulfill the requirements as far as hardnesses are concerned (1 and 2) they will, however, not satisfy the requirement as far as the shape and size stability (4) is concerned. Besides, these steel usually have a weak corrosion resistance.
Precipitation-hardenable stainless steels based on low carbon martensite, so called PH-steels, generally have the best conditions to fulfill the desired combination of features. There exist at least about twenty variants of these types of steel today. Generally it is a question of minor modifications of the three main types 17-4 PH, 17-7 PH, and 15-5 PH where the first number indicates the chromium content and the second number indicates the nickel content. Usually copper or aluminum is used as a precipitation hardening alloy additive. Generally these steels have good corrosion resistance. A review of established PH-steels, however, indicates that as a matter of fact there today does not exist any steel grade which can fulfill all the above mentioned requirements. A common disadvantage of these steels is that they usually cannot provide a sufficient precipitation-hardening effect, i.e. they cannot satisfy the important hardness condition (2).
The situation prior to the present invention thus was that there was no suitable steel available which could satisfy all the desired features.